IL-15 is a 14-15 kDa cytokine simultaneously identified by two research groups as a T cell activating factor (Grabstein, K. H. et al., Science 1994, 264, 965-968; Burton, I. D. et al., Proc. Natl. Acad. Sci. USA 1994, 91, 4935-4939) and is involved in the differentiation and proliferation of NK and T cells. High levels of IL-15 expression have been associated to the pathogenesis of autoimmune and inflammatory diseases, like in Crohn's disease (Kirman I., Am. I. Gastroenterol. 1996, 91: 1789-1794), psoriasis (Rickert R., J. Immunol., 2000, 165: 2240-2250), leukemias (YamadaY Leukemia and Lymphoma 1999, 35: 37-4)5 and rheumatoid arthritis (RA) (McInnes I. B., Immunology Today 1998, 19: 75-79; Graft rejection (Pavlakis M, transpinatation 1996, Manfro R C., Transplant Proc. 1997). Accordingly IL-15 antagonists could be a potential therapeutic to treat inflammatory diseases and several IL-15 antagonists have been described in the prior art. For example Ferrari-Lacraz S. et al. described an IL-15 antagonist consisting of an IL-15 mutant fused to a Fc domain of an immunoglobulin and demonstrated that said antagonist could be useful for the treatment of rheumatoid arthritis (Ferrari-Lacraz S, Zanelli E, Neuberg M, Donskoy E, Kim Y S, Zheng X X, Hancock W W, Maslinski W, Li X C, Strom T B, Moll T. Targeting IL-15 receptor-bearing cells with an antagonist mutant IL-15/Fc protein prevents disease development and progression in murine collagen-induced arthritis. J Immunol. 2004 Nov. 1; 173(9):5818-26.). Bemard et al. identified in 2004 two sequences of the IL-15 molecule for binding to IL15-Ralpha. Those sequences comprise amino acids 44 to 52 and 64 to 68 in the mature protein, and they also described muteins that could act as agonists or antagonists of IL-15 (Bemard I. et al. I Biol Chem 2004, 279: 24313-24322). Furthermore Pedreau H. et al. (Harmonie Perdreau; Ariane Plet; Yannick Jacques; Université de Nantes. Unité de Formation et de Recherche de Médecine et des Techniques Médicales.; École doctorale 502 Biologie-Santé (Nantes-Angers). Biologie de l'interleukine-15: de son ADN à ses voies de signalisation. 2010; Thèse de doctorat: Médecine. Biologie, médecine et santé. Immunologie: Nantes: 2010) described two IL-15 muteins for the production in bacculovirus.